Reactions to tighter hurricane intensity/SST link

There was another twist to the hurricanes/global warming issue in Science Express on Friday where a new paper from the Webster/Curry team just appeared. This study, lead by Carlos Hoyos, crosses a few t’s and dots a couple of i’s on the connection of increasing numbers of intense hurricanes (Cat. 4 and 5) to sea surface temperatures (SST). Basically, they looked at a number of other key variables for hurricane intensity (like wind shear and humidity) and examined whether there was any pattern to those variables across the different ocean basins that they study. Bottom line? None of the other variables have as much explanatory power for the long term trends as SST which is the only consistently trending constituent in the mix. So far, so un-surprising. However, one interesting aspect of this story is that almost all the key players in the ongoing debate were interviewed by different journalists in various media and those comments are probably more useful for gauging the state of play than the details of the new paper itself.

First off, Chris Landsea, a serious researcher in the ‘dissenting’ camp, in the WSJ story (reprinted here) and in a Newsday article is quoted as saying that the data (particularly from the early 70s and the Northern Indian Ocean) aren’t good enough to come to these conclusions. However, Curry riposts that for the doubling of Cat 4 + Cat 5 storms to be an artefact, half the Cat 1 + Cat 2 storms in the early records would have had to have been mis-classified Cat 4 storms instead and she thinks that unlikely. More oddly, Landsea also makes the point that the sensitivity of the hurricane intensity disagrees with model predictions and theory. This is odd, not because it’s not true (and is the principle reason why the attribution of more intense hurricanes to GW is not yet set in stone), but because Landsea has previously been much more of a champion of favoring observations over modelling.

In the same National Geographic piece, Roger Pielke Jr. is broadly supportive and makes his standard comment that the increasing damages from hurricanes is mostly related to increased development rather than changes in intensity, but that obviously, future potential increases in intensity will be an additional factor.

Fox News interviewed Kerry Emanuel (author of a related study last year, which was slightly modified following comments by Landsea in Nature), who is also unsurprised that the longer term trends are related to SSTs and not to any of the other factors. Kevin Trenberth (also in the Fox News piece) made a good point though. The authors of the study used the NCEP reanalysis as the source of their data. A reanalysis is a re-running of the current state-of-the-art weather forecasting model for all the sources of data that were available in the past (i.e. a hindcast of what all the 6 hour weather forecasts would have been if they had used today’s model). These projects (and there are two main ones – NCEP and ERA-40) have a problem in that the amount of useful data increases as you go along – most significantly around 1979 when satellite data starts to be significant. So estimates of key quantities are likely to be worse prior to 1979. Not mentioned, but conceivably important is that the NCEP reanalysis is tied in some respects to the radiosonde data, which, as we discussed last year, may have some spurious trends. This doesn’t obviously affect the results significantly, but it does suggest that doing the analyses again using the ERA-40 data might be a useful check.

So where does that all leave us? Basically, although everyone acknowledges that there are data problems early in the record, it seems clear that there has been a global rise of the most intense hurricanes over the last 30 years and the most obvious explanation is that this is related to the contemporaneous increases in tropical SST in each basin. However, the magnitude of the correlation cannot yet be explained in terms of our basic theoretical understanding, and is significantly stronger than some modelling work has suggested it should be. Possibly the theory needs work (hurricanes are a complicated business!) or there are other factors at play that haven’t yet been considered. Since the SST changes are global, and almost certainly tied to greenhouse gas driven global warming, there are the beginnings of a corroborated link between increases in hurricane intensity and GW – however, so far there are only a couple of ducks in a row.

136 Responses to “Reactions to tighter hurricane intensity/SST link”

Thank you for the update on potential influences of climate disruption on tropical cyclones.

I agree mostly with Roger Pielke, Jr. (on this one issue…); in general I suspect that for the US, the ‘rush to tropical coastlines’ has, and is likely to continue having a much bigger impact than the AGW-induced intensity rise. However, the US is not the world, and for parts of the world where moving away from the coast is not an option (e.g. Bangladesh), this is a very serious danger.

Also – even the US, should the ‘rush to tropical coastlines’ continue, it will, I fear, combine synergisticly with any AGW-induced increased in intensity.

In Chicago 2 weeks ago, the Association of American Geographers met, and the Climate Specialty Group hosted several hurricane sessions, including this talk from Davis, Knappenberger and Michaels:

Trends in Hurricane Intensity and Linkages to North Atlantic Sea-Surface Temperatures

abstract:

Several papers have suggested rising sea surface temperatures (SSTs) in the North Atlantic hurricane formation region are linked to recent increases in hurricane activity, and that the trend of rising SSTs during the past 3 to 4 decades bears a strong resemblance to that projected to occur from increasing greenhouse gas concentrations. However, none of these studies have directly linked the changes in SSTs to changes in hurricane intensity. Rather, they rely on trends in basin-wide averaged monthly or seasonal SSTs that are not necessarily representative of the local marine environment experienced by tropical cyclones. We demonstrate a direct connection between SST and hurricane strength. We match the 6-hourly positions from each named tropical system in the North Atlantic with the observed weekly averaged SST from the 1º latitude x 1º longitude grid cell that contained the storm’s center, during the hurricane seasons of 1982 through 2004. We identify the maximum SST encountered by each system prior to the time that the storm reached its maximum intensity. When we examine the relationship between SST and intensity, we find that although SSTs contribute significantly to tropical cyclone activity, their influence varies markedly over time. Further, we find a step-like, rather than a continuous, influence of SST on cyclone strength, suggesting that there exists a SST threshold which must be crossed before tropical cyclones develop into major hurricanes. These findings highlight the complex nature of hurricane development and counter arguments of a simple cause-and-effect relationship between rising SSTs and stronger Atlantic hurricanes.

I cannot remember where they have sent their paper, but it did not create as many murmurs as probably all of us were expecting. I think they identified 28.x C as a critical threshhold for SST, below which cat 3 was unlikely, but above which the distribution of cat 3-4-5 become random rather than linear as SSTs increase (at least it was unpredictable beyond the critical threshold). Something like that. Also note they use 1982-2004 data, rather than the earlier set.

[Response: Interesting… I wonder if K&M wrote the last sentence ;-) – William]

What does one make of the USGS press release:
“Century of Data Shows Intensification of Water Cycle but No Increase in Storms or Floods”http://www.usgs.gov/newsroom/article.asp?ID=1475
Released: 3/15/2006 12:13:21 PM”
[…]
A review of the findings from more than 100 peer-reviewed studies shows that although many aspects of the global water cycle have intensified, including precipitation and evaporation, this trend has not consistently resulted in an increase in the frequency or intensity of tropical storms or floods over the past century. The USGS findings, which have implications on the effect of global climate change, are published today in the Journal of Hydrology.

“A key question in the global climate debate is if the climate warms in the future, will the water cycle intensify and what will be the nature of that intensification,” said USGS scientist Thomas Huntington, who authored the study. “This is important because intensification of the water cycle could change water availability and increase the frequency of tropical storms, floods, and droughts, and increased water vapor in the atmosphere could amplify climate warming.”

For the report, Huntington reviewed data presented in more than 100 scientific studies. Although data are not complete, and sometimes contradictory, the weight of evidence from past studies shows on a global scale that precipitation, runoff, atmospheric water vapor, soil moisture, evapotranspiration, growing season length, and wintertime mountain glacier mass are all increasing. The key point with the glaciers is that there is more snowfall resulting in more wintertime mass accumulation – another indication of intensification.

“This intensification has been proposed and would logically seem to result in more flooding and more intense tropical storm seasons. But over the observational period, those effects are just not borne out by the data in a consistent way,” said Huntington.

Huntington notes that the long term and global scale of this study could accommodate significant variability, for example, the last two Atlantic hurricane seasons.

“We are talking about two possible overall responses to global climate warming: first an intensification of the water cycle being manifested by more moisture in the air, more precipitation, more runoff, more evapotranspiration, which we do see in this study; and second, the potential effects of the intensification that would include more flooding and more tropical storms which we don’t see in this study,” said Huntington.

The press release was to inform us that the paper was being published in the Journal of Hydrology….

Received 9 February 2004; revised 24 June 2005; accepted 7 July 2005. Available online 22 August 2005.

Abstract
“One of the more important questions in hydrology is: if the climate warms in the future, will there be an intensification of the water cycle and, if so, the nature of that intensification? There is considerable interest in this question because an intensification of the water cycle may lead to changes in water-resource availability, an increase in the frequency and intensity of tropical storms, floods, and droughts, and an amplification of warming through the water vapor feedback. Empirical evidence for ongoing intensification of the water cycle would provide additional support for the theoretical framework that links intensification with warming. This paper briefly reviews the current state of science regarding historical trends in hydrologic variables, including precipitation, runoff, tropospheric water vapor, soil moisture, glacier mass balance, evaporation, evapotranspiration, and growing season length. Data are often incomplete in spatial and temporal domains and regional analyses are variable and sometimes contradictory; however, the weight of evidence indicates an ongoing intensification of the water cycle. In contrast to these trends, the empirical evidence to date does not
consistently support an increase in the frequency or intensity of tropical storms and floods.”

The USGS press release comes after studies by Emanuel, Webster and Curry as if in an attempt at rebuttal, considering the timing and language of the notification.

But the study was received by the Journal of Hydrology
9 February 2004.

[dubious comment deleted – moderator]

[Response: Since this is a literature review only up to the end of 2003, it can’t be a rebuttal of the most recent work. – gavin]

Re Tim:
‘over the past century’?
I was under the impression that only for the Atlantic was there reliable tropical cyclone data prior to about 1943. Even studies confined to the Atlantic often ignore all data prior 1950, on the grounds that it is unreliable. Was their study specific to the Atlantic?

I’ve been living out in the Atlantic Ocean for the last 30 years, looking at these storms up close and personal. You’ll just have to take me on my word that ‘the ducks are in a row’. For someone like me who’s on the water all summer long, correlating hurricane intensity with rising sea surface temperatures is just about as easy as someone living on the snowline in the upper midwest for the last 30 years correlating the latitude of the winter snowline with global warming.

From the abstract:
> ” … the observed weekly averaged SST from the 1º latitude x 1º longitude grid cell that
> contained the storm’s center, during the hurricane seasons of 1982 through 2004….

I wish they’d included — if such detail exists — some info on tracking the sea surface temperature _after_ a storm passed as well as the ‘before’ temperature. I recall mention that Katrina was unusual because while crossing the Gulf “Ring Current” the deeper water pulled up by the hurricane was almost as warm as the sea surface, so the deeper water fed almost as much heat energy into the storm as the surface. It could slow down, it could even have backtracked, without running out of warm water energy.

Re #3: Now those are some interesting authors, although I’m not sure who Davis is. M&K’s current article at TCS seems to be the expected attack on Hoyos et al. One point I would make is that the relationship between large hurricanes and SSTs as such will always be a little fuzzy since the presence of warm water at depth seems to pay a large role in storm intensification (as with Katrina when it crossed the Loop Current). This is because hurricanes churn up deeper water, so if that water is warm enough and the hurricane is moving slowly it can result in some rather abrupt strengthening. Since everyone still seems to be talking SSTs, I assume there must be a lack of data about temperature at depth.

[Response: Since this is a literature review only up to the end of 2003, it can’t be a rebuttal of the most recent work. – gavin]

Certainly not.

My point is that the USGS press release headline as well as the
text did not indicate the literature review was only through 2003.

“Century of Data Shows Intensification of Water Cycle but
No Increase in Storms or Floods
Released: 3/15/2006 12:13:21 PM”
(excerpt)
A review of the findings from more than 100 peer-reviewed studies shows that although many aspects of the global water cycle have intensified, including precipitation and evaporation, this trend has not consistently resulted in an increase in the frequency or intensity of tropical storms or floods over the past century. The USGS findings, which have implications on the effect of global climate change, are published today in the Journal of Hydrology.
[…]

It was only after a lengthy search through the web that I found
the page indicating the dates
for submission to the Journal of Hydrology.

I was misled, assuming that the USGS was revealing a preponderance
of research to override the findings of Emanuel, Webster and Curry.

Laymen readers did not know that the research was dated
by more than two years. I would go so far, in light of the efforts
of the current administration, the EPA, and of late even NASA
to “manage” findings by climate scientists that someone intended
for the press release from the USGS Newsroom to keep the public
in the dark regarding this point as they proclaimed:
“Century of Data Shows Intensification of Water Cycle but
No Increase in Storms or Floods”

[Response: Another problem with a “meta analysis” of this sort is that if you throw a lot of high quality work (like Emanuel’s paper or the Webster group papers) into the pot with a lot of junk, you’re bound to get an ambiguous view about what is going on. –raypierre]

Some time ago, I pointed out that increased settlement on the Gulf coast and increased sea surface temperatures are independent of one another. Roger Pielke is asking the wrong question. It is not whether most of the increase in hurricane damage is from increased settlement. That is totally irrelevant unless you are planning to depopulate the area (putting on a tin foil hat and looking at the management of Katrina hmmm…) Whether there is an increase in hurricanes and/or their intensity is another issue.

IF you believe that the increase in damage is from societal causes, then how much would it cost to clear the coast of houses? Could this be done at all? If it cannot be done, what mitigations are possible. How would the cost of decreasing anthropic global warming compare with the cost of moving people and industry from the Gulf coast of the US? Are there other things that could be done?

If the public & policymakers get the impression that reducing GHG
emissions will reduce US hurricane damage, but, after GHG emissions
are dramatically reduced, US hurricane damage either remains stable or
continues to rise, there will be a loss of trust among policymakers
and members of the public.

If current settlement trends continue, and a dramatic increase in
hurricane preparedness does not occur, the fact that settlement trends
are the dominant factor will mean that hurricane damages will most
likely continue to increase even if GHG emissions are reduced to
zero.

The increase in hurricane damage due to increase in settlement
does matter, because it means that the allowing people and
policymakers to think GHGs are the only factor, would be a serious
mistake. Reduction of GHGs needs to be advocated, but it should not be
advocated on the basis that a reduction in US hurricane damages will
necessarily follow.

Reducing GHG emissions will have many benefits, but when it comes to
hurricanes, such action can only hope to slow or halt the rise in
intensity. This will not (by itself) reduce US hurricane damages; it
will only prevent US hurricane damages from rising as fast as they
would have if GHG emissions went on ‘as usual’.

(Aside: ‘depopulation’ is hardly the only way to cope with settlement
trends; for a society as wealthy as we are, preparedness – much, much
more extensive preparedness – is another way.)

(Second aside: As far as I know, only for the US have settlement
trends been shown to be the dominant factor in tropical cyclone
damages. This implies that AGW-induced increases in intensity play a
larger role in a global context.)

[Response:It’s perhaps ironical that the hutticane activityaffects the petroleum activity in the Gulf, which is associated with increased levels of CO2. These damages come on top of the coastal settlements. But, this is not our focus here, as we take a scientific view of tropical cyclones. The important question is: is the tropical cyclone pattern changing? Are the TCs becoming more frequent or intense? Rather than looking at insurance coasts, one could look at the rainfall associated with the storms, in addition to usual direct measurements. Since the storms are fuelled by the gas-to-liquid conversion of water, the associated rainfall amounts ought to provide a clue of the ‘fuel consumed by the hurricanes’. -rasmus]

I was at the talk, and I believe the question was raised about depth of warmth/heat content…and Davis acknowledged that it would be good information to have but it was not available for their study.

BTW, Davis is one of the World Climate Report editors/contributors…In fact he must have written their recent bit on Ken Kunkel’s extreme weather presentation, because he was the only one from WCR there.

Even more interesting, if the moderators allow this diversion, Davis very recently co-authored a paper with Real Climate’s very own Dr. Michael Mann (via their advisee, who was 1st author), and that paper won the AAG Climate Specialty Group’s paper of the year award. For those of you following the McIntyre-Mann, “x degrees of separation” game over at inkstain, I think this is a very interesting way of arriving at 4.

llewelly, “snark on” I take it you are referring to the current high level of trust and mutual respect between those in the US engaged in climate science and those in the US who are making policy /snark

However to return to serious, I think you are offering a false choice. The first question we must confront is whether increasing GHG concentrations are intensifying huricanes. The answer appear to be increasingly yes, and on the science policy side the debate is becoming increasingly strident. See for example Jerry Mahlman’s comment http://tinyurl.com/p89o2 for an extreme example of one side and almost any of Bill Gray’s comments on the other. The difference between what is occurring now and what has happened on climate issues in the past is that those on the consensus side have finally realized that it is necessary to push back against the denialists and their funders.

The second question is what effect will that have on storm damage. It is there that one sees again the tension between AND and OR. The Pielkes and Lomborgs of the world would offer a choice of either one or the other, pick one. It looks increasingly likely that to do so would produce at best a leveling off of hurricane damage into the future. This is not to mention that Gray is probably right that there will be an increase in storm intensity and number over the next 15 or so years as part of a natural cycle, so the problem is really natural cycles AND GHG diriven sea surface warming AND settlement and investment on the Gulf and Atlantic coasts in the US. It is also worth restating that in coatal areas where there has not been such a massive change in settlement patterns, the anthropic and natural effects dominate. We clearly have to take actions on many levels, but you can ruminate on the morality of the following postion: Assuming that hurricane damage in the US is driven by settlement of the Gulf Coast, the US might make a policy choice to emphasize adaptation (the cost of resettlement would be astronomical (> 10^Kyoto). To do so without taking action on GHG emissions would leave the rest of the world (or at least those parts subject to tropical cyclones) with an increasing problem.

Allow me to invert your question. What if we take actions on the coastal settlement side and things continue to get worse because of natural and anthropic sea surface warming? The real problem IMHO, is that politics operates by simplification to a single point while in the real world there are many factors that operate simultaneously. They can be addressed singly or in combination. The latter is more effective as it allows trade offs between actions. As far as the science part of it goes it is increasingly clear that GHG increases are increasing storm intensity. Nailing that down is the important thing that climate science must do at this point, so that we can move on to the policy issue of what to do about it (or not)

Bob Davis also won the AAG Climate Specialty Group Paper of the Year award in 2004 for his paper “Decadal changes in summer mortality in U.S. cities,” International Journal of Biometeorology 47:166-175. On this instance his co-authors were Pat Michaels, Wendy Novicoff, and myself.

Either Davis has an “in” with the Climate Specialty group, or he actually produces some decent stuff :^)

2, the world is seeing a rush to the coasts, and an increase in the wealth in exposed locations at rates in some places that far exceed that seen in the US. The human vulnerability is obviously much greater in developing countries

One further comment. You write, “This is odd, not because it’s not true (and is the principle reason why the attribution of more intense hurricanes to GW is not yet set in stone), but because Landsea has previously been much more of a champion of favoring observations over modelling.”

I think this misses Landsea’s point, though I have seen others make the same critique of Landsea’s views, sometimes a bit too glibly.

It is one thing to argue about models versus data in cases where the basic theory is commonly understood. We saw this occur in the satellite versus surface temperature trend debate in which at various times different sides of this debates aligned with models or data. Modelers generally support their models, empiricists the data, and in general Landsea seems to follow this tribal association as might be expected.

However, it is something altogether different in a situation where the basic theoretical underpinning of the models is under question, as you suggest in this case, “possibly the theory needs work”.

Landsea is not selectively picking models over data in this case, but saying very much the same thing that you seem to be saying — he is saying the recent studies in hurricane intensity trends, if they stand up, call into question the theoretical basis on which recent model studies are based (e.g., Knutson and Tuleya). He is not saying, as you seem to imply, that we should believe the models in this case, but that the data, if true (and clearly he thinks there are remaining problems with the data), go against the fundamental theoretical understandings on which data is interpreted and models are based. This is a case of data versus theory, not data versus model.

llewelly, “snark on” I take it you are referring to the current high
level of trust and mutual respect between those in the US engaged in
climate science and those in the US who are making policy /snark

I agree that the unwarranted suspicion many US politicians have
towards climate scientists is a serious problem – perhaps the most
serious those of us in the US face. However, I believe a mistaken
impression will surely worsen the situation.

However to return to serious, I think you are offering a false
choice.

Unfortunately, I’m not sure what choice you think I offered. The
statement about policy which I had intended to offer was: ‘Reduction
of GHGs needs to be advocated, but it should not be advocated on the
basis that a reduction in US hurricane damages will necessarily
follow.’ If you got the impression that I was offering some kind of
choice between resettlement and reduction of GHG emissions (as I think
Roger appears to offer), I apologize for being unclear. While I agree
with Roger Pielke, Jr. that settlement trends are the primary cause of
increases in US hurricane damages, I do not agree that
resettlement should be pursued ‘instead of’ reduction of GHG
emissions. However much resettlement might reduce US hurricane
damages, it will have little effect on many of the other dangers of
AGW. (Such as changes in precipitation, and changes in growing season
length.) Resettlement of US residents will also have no effect on
tropical cyclone damages in the rest of the world. I believe that if
resettlement is feasible, it can only play a partial role in a
solution to US hurricane damages. I suspect we agree on these last few
points.

As for the rest of your post, I don’t yet have much to say, except to
thank you for pointing me at your blog.

We cannot forget another simple factor…the relationship between global warming (GW) and increasing hurricane damage due to incresasing storm surge.

GW raises sea levels, mostly by thermal expansion. The higher the sea levels, the more damage most land-falling hurricanes will do with storm surge alone (unless it is in an ocean area where sea levels are not rising as much as others).

Statistically, it is storm surge that does the most hurricane damage. This makes most land-falling hurricanes more damaging than 100 years ago, all other things being equal, no matter what is happening to maximum hurricane intensity.

Secondly, land-falling hurricanes will most likely continue to do increasingly more damage in the next fifty years at least, all other things being equal, whether they are getting stronger or not due to greater GW-induced storm surge.

As an aside, I am experiencing something that I never expected to see in my lifetime. Because of the special nature of my job and my wife’s businesses, I have in the last year personally met in Boulder Colorado, eight separate people (most with families), seven of them solidly middle class, who have left New Orleans, Florida and Texas, many voluntarily, some due to forced evacuation they say, due to hurricane experiences they have had.

18, you have mis-stated my views. I do not offer GHG reduction and reducing vulnerability as “either/or,” but as an “and” —

“But a great irony here is that invoking
the modulation of future hurricanes to justify
energy policies to mitigate climate change may prove
counterproductive. Not only does this provide a great
opening for criticism of the underlying scientific
reasoning, it leads to advocacy of policies that simply
will not be effective with respect to addressing future
hurricane impacts. There are much, much better ways
to deal with the threat of hurricanes than with energy
policies (e.g., Pielke and Pielke 1997). There are also
much, much better ways to justify climate mitigation
policies than with hurricanes (e.g., Rayner 2004).”

[Response: This statement does seem to offer an AND, but I think maybe it is a different sort of AND than many people have in mind. My reading of this statement is that you are saying that the likelihood that global warming is increasing the destructive potential of hurricanes (and is likely to do so increasingly in the future) is irrelevant to the policy debate about hurricane damage. I read this as saying there’s plenty to be alarmed at when thinking about AGW, but hurricanes aren’t one of them. I don’t find that a tenable position (if that’s indeed what you’re saying) because the fact that the increase is detectable at all opens the possibility that hurricane strength will increase much more than models currently predict. In your BAMS article, you seem to draw comfort from the rather modest increase yielded by 3D mesoscale models, but are discounting the possibility that Emanuel’s observational analysis really is suggesting an intensification mechanism that isn’t yet understood. When you combine the possibility (if not probability) of severely intensified hurricanes in 2050 with the unlikelihood that coastal settlement patterns will change much, you do indeed get something related to AGW that needs to be factored into the mix. Nobody ever said that hurricanes are the main reason we should do something about AGW, but people are absolutely right to take hurricane effects as an example of the changes being wrought on the climate, and to be alarmed as a result. –raypierre]

Considering that my this paper that was formally published on March 13 2005 was submitted in February 2004 and was accepted in final form in July 2005, and considering that I am not clairvoyant, I don’t think I should be faulted for not having included the many papers published on this topic in 2005 and 2006. The lengthy delay in getting papers published at the Journal of Hydrology is simply out of my control and I do not think it is incumbent on the USGS to add a disclaimer on every press release that additional papers have been published since the acceptance of the paper that either support or are contrary to its findings.

Note that it is simply incorrect to state that the compilation of papers extends only through 2003, if you look at the report you will see that there are 18 papers cited from 2004 and 5 papers cited from early 2005 (including Trenberth et al. that was cited as in Press at the time).

If I were to submit this review today I would make certain changes to accommodate newly published research.

In the paper I state that there is no evidence that storm frequency, intensity, or duration increased in the 20th century. However, since that paper was submitted in early 2004 and was “In Press”, more than a year ago, recent papers by Emanuel et al., 2005, Nature 436:686-688; Webster et al. 2005 Science 309:1844-1846, and Hoyos et al. 2006 (Released in March 2006 ScienceExpress) have emerged reporting trends of increases in storm intensity.

With the benefit of these new reports, I wish I could have cited those recent papers for observations of trends, AND, by the way, I probably should have also cited (Landsea et al 1999; Climatic Change 42:89-129; Goldenberg et al., Science, 2001; 293:474-479, and Chan and Liu 2004 J. of Climate 17:4590-4602) for reporting lack of trends. And, now I would cite Pielke et al. 2005 BAMS Nov. 2005 pages 1571-1575 as a review of the issue that places these recent papers in context. In light of these recent papers I would change the language to state “the evidence does not consistently support an increase in the frequency or intensity of major storms”.

To my mind the more important result of this review paper was in the synthesis of information from many discipines that point towards an ongoing intensification of the hydrologic cycle — not that I did not find evidence for more frequent or intense storms. I believe that hydrologic responses to cimate change other than “extreme events” have not received the attention that they should have either in the IPCC or other assessments, in part because of the attention directed towards “extreme events”. It is ironic, therefore, that the feedback that I have received on the J. Hydrol paper has been almost exclusively with the findings on “extreme events”.

While on the topic of updating the paper with recent findings (for those interested in anything other than extreme events) I would add the following notes.

If I were to publish this paper today I would also cite Milly et al. 2005 as a very recent global modelling study that supports the argument that the global water cycle has intensified.

I would also include Schwartz et al. (2006) for further evidence of lengthening of the growing season and greater ET. I would also cite Phillipona et al. 2005 for further evidence of water vapor feedback. I would also cite Soden et al. (2005) for evidence of increasing water vapor in the upper troposphere, I would also cite Held and Soden (provided it is accepted for publication) (now submitted to the Journal of Climate) for findings consistent with increasing tropospheric water vapor. I would cite Kundzewicz et al. (2005) for lack of evidence for an increase in flooding. I would also cite Johannessen et al. (2005) for increase in winter precipitation over Greenland. I would also cite Zwally et al (2005) for evidence from the Greenland Ice Sheet and the East Antarctic Ice Sheet mass balances that are consistent with increasing winter precipitation and warmer temperatures.

[Response: Thanks for the comments. I share your frustration at what does and does not get picked up on, but I think you will find that the media (in general) are quite predictable. It is sometimes then possible to ‘head them off at the pass’ (so to speak) by crafting press releases with that in mind. -gavin]

If there is some way of quantifying the difference in damages between Cat 4 and Cat 5 events this could help make the financial case for GHG abatement. The long tapering off period doesn’t help though. A Queensland farmer said his crop was designed to resist strong winds but Cyclone Larry was too powerful. Governments seem to be wondering if they have to budget for emergency aid on a more regular basis. People are starting to join the dots.

Thanks for the first hand insights into your paper. I am puzzled by the paradox that you observe an “ongoing intensification of the hydrologic cycle, but not an increase in storm frequency and intensity”. I cannot envision physically that the hydrologic cycle can intensify and there not be an increase in storm frequency and intensity of floods and droughts. The glacier mass balance record in alpine regions does indicate a steepening of the gradient. That is a greater difference from the terminus to the head of the glacier. However, there is not a consistent rise in alpine glacier winter balance to offset the increase in summer melting. Hence the increasingly negative glacier mass balance record that you are familar with.

The hydrologic cycle can change because of changes in plant cover — it’s a common observation that after a forest fire or logging, streams and springs will flow higher and longer for a period of years. I recall hearing somewhere that removing invasive Eucalyptus species is having the same effect in areas of drought in Africa.

(Nitrogen is a big problem in native plant areas because it’s one of the limiting nutrients, and when it’s supplied whether by fertilizer or grazing animals or rainfall, that tips the balance toward European and Asian annual grasses that are very shallow-rooted — they steal every drop of rain that falls, and then burn by midsummer when the N. American native perennials are just setting seed; after a few years the deep-rooted perennials are gone; after a few more years so is the topsoil).

That’s just an amateur’s recollection, but I am sure that the climatologists can get a lot of info from the soils scientists about various known ways that streamflow can be changing that are not directly outputs of changes in storm frequency or intensity. And of course the rates of change may be changing.

Re 25, Mauri Pelto:
Storm frequency and intensity are noisy signals. Intensity is also difficult to measure. To get a rough idea of how noisy it can be, see http://www.aoml.noaa.gov/hrd/tcfaq/E11.html , which has an 1851 to 2004 record of tropical storm activity in the atlantic. (Note: the pre-1950 years in that list are of limited reliability.)

I believe they sent the paper into GRL. It’s an interesting bit of work, but in many respects it is just as limited as the Kerry Emmanuel paper they criticized, just in the opposite direction. Emmanuel looked at basin averaged SSTs (err on large scale) and Davis et al looked at single gridpoint SSTs (err on small scale). Since hurricanes are probably pulling energy from the SSTs over a scale larger than Davis and smaller than Emmanuel, there is probably some information lost/obscured in both cases.

OK, I’ll bite. The handwaving is fine and good, but we can do better. Give me a quantitative estimate of the effects that GHG reductions can have on future hurricane impacts. Here is my framework for exactly this calculation:

Give me a scientific basis for understanding what role you think GHG reduction policies should play in hurricane disaster mitigation. I get lots of criticism of my work, but the critics never offer any alternative analyses, so here is your chance ;-)

Thanks.

[Response: Your paper sets up a false dichotomy regarding controlling hurricane damage by vulnerability control vs controlling hurricane damage by GHG reductions. It presupposes that one is faced with a decision between controlling GHG just for the sake of hurricanes, and forgetting about GHG’s and spending money on bribing people to live further from the shoreline. (In your 2000 paper, you seem to make this point more broadly; you state explicitly that you think that climate impacts are best addressed through adaptation rather than prevention. I’m not saying that’s a defensible position based on your analysis, but that is indeed what you state, begging the question of why you go on to imply that it’s worthwhile to control GHG anyway). In reality, the decision is not at all like your false dichotomy. GHG’s have many impacts, increased hurricane damages being one among many. So, your comparison of the climate-related component of increased hurricane damages to the vulnerability component is irrelevant, since nobody is talking about controlling GHG’s for the sole purpose of reducing hurricane damages. Seen in this light, the public attention to the Emanuel and Webster papers is not at all misplaced. It’s part of the whole spectrum of GHG effects that need to go into the assessment of the nature of the threat. To be sure, hurricane damage has high “availability” as a threat, since people can picture it more easily than extinction of some invisible mycorhyzae, but that doesn’t make it irrelevant — it just means that some other threats are under-appreciated. For that matter, it wouldn’t take much tweaking of your “high end” climate damage numbers to make the costs look more alarming: what if the damage function turns out to be quadratic rather than linear? What if Emanuel’s observations turn out to mean that the effect of SST on hurricane intensity is larger than the theoretical prediction? What if we recognize that under business as usual CO2 won’t stop at a doubling, but go on eventually to a quadrupling or worse? None of this says that one should neglect spending on reducing vulnerability, but the clear message I take away from your writing is that people should just calm down and forget all about the effect of AGW on hurricanes. Why wouldn’t people care about such a massive sign of human impact on nature? By the way, in your arithmetic example at the above link the damages are all monetized. Could you clarify how loss of life is figured into these numbers? You make a 10% increase in hurricane damage look small compared to the increase in monetary damage due to economic factors, but comparing a 10% increase in lives lost (as an example) with a 100% increase in costs of McMansions on the shoreline floating away on the flood is not necessarily appropriate. –raypierre]

Re 28, Ben:
In addition to the concerns you list, intense hurricanes normally cause some upwelling of deeper waters, and if they are slow-moving, thus somewhat dependent on the temperature of deeper waters.

Who made the decision to change the title to include “but No Increase in Storms or Floods”?

My career (1976-2005) was hydrologic modeling and flood prediction for the Midwest and Great Plains. I was with NWS Kansas City River Forecast Center from 1976-1979, then and moved to Minnesota to help start up NWS North Central River Forecast Center. The summer “Great Flood of 1993″ in the Mississippi River basin went on for three months. Other very large floods in the Midwest were in 1997 and 2001. Runoff into Devils Lake in North Dakota has been excessive since 1995.

I understand that the Midwest is not the world, but from what I’ve seen happening in the Midwest and many other places throughout the world it seems to me to be counter to your conclusion of No Increase in Floods. Unfortunately, I have not read more than the press release about your work. I should add too that I was not allowed at work to study or talk about trends in frequency and magnitude of floods or trends in temperatures and precipitation, due to the “highly political and controversial subject of global warming”. I encountered “great difficulties” from Jan of 2000 until July of 2005 as a result of my concerns with climate change effects on hydrologic modeling and flood prediction. I removed as a federal employee in July of 2005 for objecting to not be able to study or talk about climate and hydrologic change while at the office.

Did you encounter obstacles or “firm suggestions” in coming up with and doing your study and report? If so, could you share them with us?

‘But lets be slightly more realistic, how about Kyoto? Assuming that the effects of GHG reductions on hurricane intensity are instantaneous and exactly proportional to emissions concentrations (also dubious assumptions, but lets go with them) under full and successful implementation of Kyoto, including the participation of the US, the reduction in projected damages would be about $0.03.’

Three cents on the dollar doesn’t sound like a lot. However, multiply this by a billion in the case of such a hurricane (i.e. the hurricane causes a billion dollars in damage) and you get 30 million dollars.

Take an average number of storms (6) in a given year, with, say an average of 50 million dollars in damage. There’s 300 million dollars in damage, which could bave been roughly 10 million dollars cheaper from which to rebuild.

Try increasing the savings from hurricane damage in the future with every ppm of CO2 that is reduced on a yearly basis. Billions of dollars will be saved! Now that sounds like a lot, eh?”

[Response: And also factor in that reduction in hurricane damage is not the only, or even the major, benefit of GHG control, and perhaps also the fact that it’s not entirely clear that a smart implementation of Kyoto would actually “cost” anything, given ancillary benefits of energy efficiency and health benefits from reduced or cleaner coal burning. –raypierre]

Gavin’s post is a good one, especially ending on the note of there being problems with theory. As far as that goes, I find it interesting to note that in the wake of TC Larry, Australia’s CSIRO is apparently projecting stronger storms but also *fewer* of them in the future.

Furthermore, this is consistent with a recent modeling study out of Japan, except that the Japanese study projects an anomalous frequency *increase* in the Atlantic even though frequencies elsewhere go down:

I have written up a blog post about this question of whether GW could cause stronger storms but less of them, except for in the Atlantic, but I’m no scientist. I’m just going on what I hear. Would be great if the minds at Real Climate could weigh in on the matter.

From a strictly non-policy oriented point of view, using hurricanes to ID a global warming trend is a bit like using dinosaur fossils to determine the Cretaceous-Triassic geological boundary. Hurricanes are big, relatively rare, but of great interest to the general public – like dinosaurs. There are certainly better indicators of global warming trends – ice sheet volume, sea ice extent and sea surface temperatures all come to mind – but hurricanes get people’s attention.

From a media point of view, the most irritating mis-statement I’ve seen is “Scientists state that no single event can be determined to be due to global warming.” This was at the end of a news article, and gives a misleading impression. A rather different way to say this would be “No single event can be seen as definitive proof of global warming – rather, a trend or pattern must be identified by analyzing a large number of individual events.” The fact that hurricane events are relatively rare leads to a limitation in the amount of data available – fewer events, and that’s why the ‘skeptics’ have to rely on statistical rather then mechanical arguments (notice also that the media seems to avoid any mention of the fact that hurricanes operate as ‘heat engines’).

Another statement that seems unsupported is “the natural cycle of hurricane activity is increasing”. It would be okay if it was just “hurricane activity is increasing”. This ‘natural cycle’ is based on what data?1999 time series analysis paper is one citation, but this kind of time series analysis seems highly questionable, at least. Correlation with sunspot activity? They also say that “Time series analysis offers a rigorous approach for extracting underlying cycles in a set of observations” – interpretation: it’s a bit better then eyeballing a chart – but look: fancy math! Furthermore, this paper deals with hurricane frequencies – which are supposedly? unrelated to SSTs. In any case, the idea that there is a ‘natural cycle’ of hurricane activity that is increasing due to ‘global oscillations’ seems unsupported.

Finally, the physics of a hurricane system is complicated, but takes place within the larger atmosphere-ocean system. What I’d like to know is specifically why hurricane intensity and frequency wouldn’t both be expected to increase, given that the SSTs play a primary role in hurricanes? Taking a look back at the Katrina ‘event’, if the hurricane hadn’t encountered a tounge of warm water in the Gulf (perhaps a separate ‘event’) it wouldn’t have amplified as it did. The depth of this warm layer was also critical – not just an SST effect. In the progression of tropical disturbance > tropical storm > hurricane the available energy is limiting the growth of the system; unless the vertical structure of the hurricane engine is blown apart by winds, the thing will grow when it hits a warm deep water zone.

The media has done a very poor job of reporting on this issue. The reports are dominated by the phrase ‘experts say’ and little actual explanation is attempted. Painfully obvious is how it all seems.

Chris, #35 — note the Japanese model you quote says increasing storms in the _North_ Atlantic. That would seem consistent with the heat transport and warmer sea surface temperatures — are they saying the Atlantic hurricanes will last longer as they go farther north in a warmer environment, so ‘more storm in the North Atlantic’ is explained?

Re SBerg (34). You need a convincing case to justify actions that will cost billions – that will potentially be diverted from causes for which there is zero doubt. The war in Iraq is an example of a decision based more on policy than evidence, which resulted in many bodybags on all sides. Many of us who were skeptical of the evidence for WMD at the time (and ridiculed for it) have since been proven correct. (Off-topic? I’m not the first to make a comparison to war/terror).

A decision to act on AGW theory/evidence (take your pick) is not without risk, as many seem to assume.

Perusing the Japan study I cited, the answer to your question is this:

“A comparison between the future and present-day experiments reveals that the occurrence number in the future experiment is generally reduced, showing approximately 30% total global reduction, as stated before. The reduction trend remains evident across the Northern Hemisphere, Southern Hemisphere, North Indian Ocean, western North Pacific, South Indian Ocean, and South Pacific, at the 99% level of statistical significance (based on the twosided Student’s t-test). On the other hand, a tendency of increase in the future experiment is found in the North Atlantic Ocean, where the occurrence number increases by about 30%. Though the reason for this contrast in the tendency is not presently clear, a clue may be obtained by investigating the change in the sea surface temperature (SST) between the two experiments….We can see that a positive region of SST spreads over the southeastern part of the North Atlantic Ocean. The region approximately corresponds to the area of generation for most of the hurricanes that approach and threaten the eastern coast of the United States. Of the several signatures of such warm tongues over the globe, the one over the North Atlantic is conspicuous in size, and, more importantly, coincides with the tropical cyclone source. In addition, the accompanied difference in the rainfall amount, which is shown in the lower panel of Fig. 3, exhibits a trend of increase, indicative of activated convection. This should help “seed” the tropical cyclone generations. These features can be responsible for the increased frequency of tropical cyclones over the North Atlantic Ocean.”

Our paper has been submitted to GRL and is still in the review process.

Our basic conclusions are that:

1) Encountering a SST of about 28.25C is necessary for an Atlantic tropical cyclone in order to become a Cat 3, 4, or 5 storm.

2) however, encountering SSTs greater than 28.25C doesn’t lead to stronger storms (i.e. there is no significant relationship between SST and intensity at SSTs above 28.25)

So, rising SSTs will lead to a greater number of major hurricanes (up until SSTs rise enough that all storms encounter 28.25), but not stronger major hurricanes.

This result is similar to that which is reported in Webster et al. (2005), where they find: “This increase in category 4 and 5 hurricanes has not been accompanied by an increase in the actual intensity of the most intense hurricanes. The maximum instensity has remained remarkably static over the past 35 years.”

Further, we found the SST increases alone since 1982 (the start of the high resolution SST data) cannot explain all of the increase in the number of major hurricanes. Thus we conclude that other factors have also contributed to the observed frequency increase–a result that is supported by the results of Hoyos et al. (2006), at least for the North Atlantic.

I am hopeful that our paper will be accepted at GRL and then look forward to a discussion of it on this group.

I have some further thoughts on the implications of Hoyos et al. that I hope to share with you all–and to hear your thoughts on them.

>41 – Ah. Chris, I was actually thinking (probably NOT in line with proper use of the words) ‘North’ vs. the near-equatorial Atlantic. I expect you’ve got the naming of the Atlantic correct. I’ll wait on the experts to clarify the mechanism(s) if they have them to suggest.

Oops, yes, thank you, the KT boundary is the Cretaceous-Tertiary boundary not the Triassic. I was trying to refer to the large extinction event that occurred some 65 million years ago; if you wanted to date the event using the sedimentary record it would be better to look at a widespread, well-preserved fossil type – a marine snail or something – rather then dinosaurs. The extinction of the dinosaurs (and a host of other species) is now generally blamed on meteor strike-induced climate change.

Dinosaurs link up to the current situation in other ways; some deceased swamp dinosaur carbon is probably going up a coal-fired power plant smokestack right this very moment. Yes, coal – the highest carbon output per energy unit of all the fossil fuels, and the source of 86% of electricity-related CO2 emissions in the US. Readers might want to look here for the EIA 2003 report: 2003 CO2 emissions

Ike, I thought the majority of the world’s minable coal reserves were laid down during the Carboniferous? Probably some coal was laid down during the Cretaceous, but given the Cretaceous had much lower carbon levels than the Carboniferous, I suspect much less. (Here I’m assuming more CO2 in the atmosphere means more carbon in the biosphere – is that reasonable?)

“SSTs have been increasing since 1970, as have major hurricanes, but the connection is not nearly as simple as some authors are suggesting. And careful scrutiny of ALL of the available data shows the connection to global warming is less than tenuous.”

To all appearances, this statement even contradicts your own recent paper. What’s up with that?

As early as 1992 my Earth Day display on GW had a big hurricane on it, with mention of GW contributing to hurricane intensity AND frequency. I’m not gloating that I was right — I wish I had been proved wrong.

RE FREQUENCY. Some people have suggested that if SST (& GW) is increasing hurricane intensity, it seems logical it would also be increasing hurricane frequency (all other things being equal – which I suppose they may not be), since the storms are moving up in category: 1 to 2, 2 to 3, etc., then some “tropical storms” would also be moving up to Cat. 1, thereby increasing hurricane frequency.

I thought of some way in which you could get more intensity, without increased frequency: Perhaps hurricanes & tropical storms are clubbing together into more intense storms (what would have been 2 hurricanes given lower SST, become 1 more intense hurricane, given the higher SST).

OTOH, it might just be that there aren’t enough numbers to reach .05 significance on frequency, not that we can say for certain (at .05 sig) that frequency is NOT increasing. In other words, frequency might be increasing, but our statistical operations are what’s failing to tell us that (due to low sample number, not lack of association).

WRT 42. Based on a relatively recent EOS article setting forth the idea that it is not only SST but also the depth profile of the warming that effects hurricane strengthening in the Gulf, would it not be possible that additions Carribean heat content could warm the sea lower down, increasing intensity.

[Response:I think that both high SST and that high temperatures need to extend some distance downward are requisites for hurricanes. The SST can be viewed as important for the instant processes, whereas the upper surface layer heat capacity determines what will be the subsequent SSTs. Hurricanes stirr up the sea (mixing or Ekman pumping), and if there is a thin warm surface layer, colder water underneath will be brought up, and hence give rise to lower surface temperatures (SST). There is also rain associated with hurricanes, falling over the local area, but I have not seen much discussion on how the rain water may affect SST/surface layer heat content – it’s probably not very important. The vertical temperature profile may also play a role, as warmer water is lighter, and thus the stability of the water column depends on how fast the temperature drops with depth – more stable water column is less prone to mixing. For the Ekman pumping (one good example of the Ekman pumping is the so-called ‘cold tongue’ near in the eastern part of the Pacific near the Equator), the stability is not so important, but rather the depth of the upper warm surface layer. Addition of heat content could result in higher temperatures deeper down, but this depends on the process. Warming from above, such as solar heating, depends on the penetration of the radiation. The lighter warm water tends to stay near the surface and sensible heat conduction plays little role (as opposed to advection, which is heat transport due to fluid motion). Over time, however, the heat may diffuse downward. Often the depth of the warm upper surface layers is influenced by ocean dynamics, meaning long and slow waves which are seen as slow undulation of the interface between the upper and deeper layers. These (Rossby) waves are also related to currents, because they set up pressure differences (geostrophic motion). It is believed that such waves play a key role in initiating and terminating El Nino events. I suppose it’s fair to say that the issue regarding SST and heat content is fairly complicated. -rasmus]

Does all of this really matter? AGW is happening and mitigation activities are needed regardless of what is going on with hurricanes. Shades of the hockey stick!

Every intuition tells me that GW should affect hurricanes. However, my intuition is not science. I am concerned with the possibility that some of the very excellent scientists on this site may have foregone their science and succumbed to a similar intuition.

I would assume that NOAA is at least mainstream, if not authoritative. A four month old article in their magazine states, “… the tropical multi-decadal signal is causing the increased Atlantic hurricane activity since 1965, and is not related to global warming”. Then come tongue in cheek papers by a couple of researchers who state that GW may affect intensity if not quantity. But another researcher reports in this thread (#42) that his work may refute that. All of this is early work and certainly should not throw over all of the work performed that lead to the NOAA conclusion.

Yet here I read that that scientists such as Landsea and Gray, who belong to the NOAA viewpoint, are “dissenters”. Is dissenter the same as skeptic or denier? Are you not turning things on their head? Who is the “mainstream” voicing “accepted” science and who is the dissenter?

I would appreciate having this clarified. I would not like to see the capable scientists on this site making themselves vulnerable to the same characterization currently reserved for AGW skeptics. The risk to reputations among those in the non-scientific aspect of AGW is not worth it, particularly on a topic that does not matter. Is it intuition or solid science that is speaking?

[Response: There is a lot of debate on this issue, and definitive statements one way or the other are (at present) unsupportable. The NOAA ‘official’ claim of no GW influence was in many people’s opinion too definitive a conclusion and, in fact, NOAA has since modified its public stance to acknowledge the debate even within the agency. This is actually a good example of genuine disagreements on valid issues (for the most part) which contrasts markedly with the manufactured ‘controversies’ that keep coming up from the various contrarians. In this issue there is no real ‘mainstream’ – unless you consider ‘undecided’ a position – though some individuals have made clear their preferences for the interpretation. – gavin]

Steve: I think the results from Hoyos et al. fit nicely with our GRL results–SSTs are only part of the equation (at least in the North Atlantic–and neither paper makes it clear that anthropogenic effects are the primary cause of the warming of Atlantic SSTs during the past few decades.

Re: #48: We didn’t incorporate any ocean data other than SST in our analysis, so I can’t really comment on the effects of a warming at depth. I am not familar with the EOS article, so I’ll have to look into it. Thanks for bringing to my attention.